The present invention pertains to the electical power management art and, more particularly, to an improved load shed timer.
Timers utilized to activate load shedding relay in utility load management systems are well known in the power management art. In such systems, a central controller monitors electical power being consumed by a plurality of load sites. During periods of peak power consumption it is desirable to disconnect from, or "shed" certain deferable loads from the system. This is accomplished by providing at the load sites a means to interrupt power to deferable loads, such as water heaters or air conditioners, for a given time interval. Generally, a command signal is transmitted by the central controller indicating the need to shed loads. This signal is received by a receiver at the remote load site, thereafter activating a timer which "times out" the load for a given time period. It is perferable that the time out period be between certain minimum and maximum time limits but, more importantly, it is desirable to make the time period totally random between these limits such that multiple loads do not switch back onto the system simultaneously thereby generating a large instantaneous increased power demand on the power generating equipment.
Two basic load shedding timer configurations have been known in the prior art. The first incorporates a standard RC timing circuit. Here, upon receipt of a shed command from the central controller, the load is taken off the system for a time determined by the voltage across a capacitor being charged through a given value resistor. Variations in the value of resistance and capacitance are relied upon to bring the loads back onto the system at varying time periods. It has been found, however, that despite variations in values of resistance and capacitance from one load site to the next, such systems result in clusters of load sites coming back onto the system within a very short time interval, resulting in a high differential power demand.
The second prior art shedding timer utilized a digital counter. Here, upon receipt of a shed command from the central controller, the counter is fed a clock signal causing the counter output to increment. By connecting the shedding relays in the load sites to different counter outputs, a systematic arrangement can be effected whereby a limited number of sites come back onto the system at a given time. Nonetheless, this system has proved undesirable in that groups of sites simultaneously come back on system at different incremental periods, generating a staircase demand curve.
In addition, the prior art shedding timers suffered the further problem that those users whose timer interval is the greatest always suffer the longest interruption.
Thus, there has been a long felt need in the power management art for a timer which restores load sites in a linear manner between minumum and maximum time limits in order to prevent the generation of large differentials in the power demand curve.